Typically, a solid oxide fuel cell (SOFC) employs a solid electrolyte of ion-conductive oxide such as stabilized zirconia. The solid electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, normally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
As the fuel gas supplied to the fuel cell, normally, a hydrogen gas produced from hydrocarbon raw material by a reformer is used. In general, in the reformer, a reformed raw material gas is obtained from hydrocarbon raw material of a fossil fuel or the like, such as methane or LNG, and the reformed raw material gas undergoes steam reforming, for example, to produce a reformed gas (fuel gas).
The operating temperature of the fuel cell of this type is relatively high. Therefore, the exhaust gas containing the fuel gas and the oxygen-containing gas partially consumed in the power generation reaction is hot. In most cases, the exhaust gas of this type is discarded wastefully. Thus, it is desired to effectively utilize the heat energy.
In this regard, for example, a power storage type heat electricity combined supply system disclosed in Japanese Laid-Open Patent Publication No. 06-176792 (hereinafter referred to as the conventional technique 1) is known. In the system, as shown in FIG. 6, as heat supply systems, an absorbed water heating system for an exhaust heat boiler 1a, a heating system for an Na-S battery (sodium-sulfur battery) 2a, a heat supply system by a boiler 3a are provided. Further, as power supply systems, a power generator 4a, the Na-S battery 2a, and a commercial power system 5a are provided.
The absorbed water heating system as one of the heat supply systems includes a water-water heat exchanger 7a for cooling bearing coolant water of a gas turbine 6a or the like and a gas-water heat exchanger 9a provided in an exhaust air system for a constant temperature bath 8a which keeps the Na-S battery 2a at a constant temperature.
Absorbed water is heated by the water-water heat exchanger 7a and the gas-water heat exchanger 9a, and then, supplied to the exhaust heat boiler 1a. The heating system for the Na-S battery 2a is branched at the outlet to guide the hot exhaust gas from the internal combustion engine such as the gas turbine 6a into the constant temperature bath 8a through a heat insulation pipe, and to heat the Na-S battery 2a for keeping the Na-S battery 2a at the constant temperature.
According to the disclosure, excessive electrical energy is charged to achieve effective utilization of generated electricity, and power storage equipment for charging and discharging electrical energy as necessary is provided. Thus, since the hot exhaust gas which is already available is used as a heat source of the battery (Na-S battery) which needs to be kept at high temperature for operation at high temperature, the expense for the facilities such as conventional electric heaters and electrical energy required for the heaters can be reduced.
Further, in a load following type fuel cell power generation system disclosed in Japanese Laid-Open Patent Publication No. 2002-334710 (hereinafter referred to as the conventional technique 2), as shown in FIG. 7, a fuel cell stack 2b is connected to a desulfurizer 1b as a fuel processing device, and an inverter 3b is connected to the fuel cell stack 2b. A sodium-sulfur battery 5b is connected between the fuel cell stack 2b and the inverter 3b through a bidirectional converter 4b. 
A heat exchanger 6b connected to the fuel cell stack 2b through a coolant water pipe is provided. Further, a heater 7b for heating the sodium-sulfur battery 5b is provided. The heater 7b is connected to the heat exchanger 6b through a medium pipe for heat transfer, and a hot water supply unit 8b is connected to the heat exchanger 6b through a medium tube for heat transfer.
In the structure, electrical power outputted from the load following type fuel cell power generation system can be changed in accordance with the electrical power required for the load (load power) by charging and discharging the sodium-sulfur battery, and thus, can be changed to follow rapid fluctuations in the load power. According to the disclosure, since it is not required to heat the sodium-sulfur battery using electrical energy, no losses in the electrical energy occur.